As far as I know, that really depends on the operating system and how it handles open file handles. However, most of the time it will buffer the file--read in one line at a time. I wouldn't worry about it.

Just don't do something like this:

open(FILE, "somefile") or die $!;
my @log = <FILE>; # read entire file into RAM
close(FILE);

foreach my $line (@log) {
    # do stuff to $line
}
[download]

That will definitely clutter up your machine's memory!

On the other hand, if you have a gigabyte of RAM and you WANT to load the entire file instead of using slow disk accesses, knock yourself out. =)